This Phase III clinical trial evaluates the possible benefit of adjuvant chemotherapy (5-FU and Leucovorin) in patients with resected gastric adenocarcinoma. The specific objectives are to compare overall and disease free survival between patients being treated with surgical resection only and those being treated with surgery plus adjuvant therapy; to compare the incidence and patterns of disease failure between surgery and surgery plus adjuvant therapy; and to assess patient tolerance of upper abdominal chemoradiation after gastric resection. Nationwide 486 patients have been accrued as of 12/31/96. Of 187 patients evaluated for toxicity, 54 (29%) experienced grade 4 hematologic toxicity. Three patients died from treatment-related causes: infection (2) and respiratory arrest.